It has a big atmosphere. About 10 times as much per square meter of
surface as Earth. It is mostly nitrogen. How can that be, with so little
gravity? It helps a lot that Titan is so cold, so the gas molecules
aren't moving very fast. That makes it just possible for Titan to
keep its atmosphere.

Still, I have not seen a convincing explanation of why Titan got so
much atmosphere to start with. Presumably, there was a considerable amount
of ammonia ice, NH3 in the original Titan. Then a little warming
released ammonia gas into the atmosphere, where ultraviolet radiation from
the sun broke it into nitrogen and hydrogen (which mostly escaped).

What is the atmosphere made of?

Mostly N2.

Some methane, CH4.

Hydrocarbon smog ( C2H2, C2H4, C2H6, C3H8 seen).

Ethane (C2H6) should condense and rain onto surface.

An ethane ocean?

Not many details are known, but it would clearly be an interesting
destination for a probe.